Conventionally, centrifugal projectors and nozzle projectors have been known as projectors used in shot blasting, shot peening, and the like. A centrifugal projector is an apparatus which utilizes centrifugal force. A nozzle projector is an apparatus which utilizes air pressure. Nozzle projectors are efficient when the projection range is narrow in width, but are not suited to situations where the projection range is wide.
Centrifugal projectors are efficient when the projection range is wide, but were inefficient and ill-suited to situations where the projection range was narrow. In other words, in centrifugal projectors, it was difficult to concentrate the projection pattern and raise projection efficiency. Here the term “projection pattern” means the distribution of the percentage of the total amount of projection material projected at the product (processing target) hitting each position thereon. Also, “projection pattern” indicates what percent of the total projected amount is projected in a 360° range at predetermined angular positions in the circumferential direction around a rotary shaft. In the description below, the former meaning is used in explaining FIG. 13, but in other parts both the former and latter meanings are used. In addition, centrifugal projectors have better acceleration efficiency than nozzle projectors, so it is desirable to concentrate the projection pattern using a centrifugal projector to increase projection efficiency.